Download Newton`s Three Laws of Motion

Newton's Three Laws
of Motion
by
BUENO OLIVIER
Isaac Newton (1642-1727)
 Life & Character
– Born at Woolsthorpe in
Lincolnshire (England)
– entered Cambridge
University in 1661
– Professor of
Mathematics in 1669
and Natural
Philosopher
– President of the Royal
Society of London in
1703 until death.
Scientific achievements
– OPTICS
– discovered measurable,
mathematical patterns
in the phenomenon of
color, found white light
as mixture of infinitely
varied colored
rays,…book: Opticks
(1692).
 MATHEMATICS
– discovered general
methods of resolving
problems of curvature,
embraced in his
"method of fluxions"
and "inverse method of
fluxions",..books:
Principia I and II
(1687)
Scientific achievements
 GRAVITATION
– calculated the relative
masses of heavenly
bodies from their
gravitational forces,
calculated the force
needed to hold the
Moon in its orbit book:
Principia I and III
(1687)
 MECHANICS
– calculated the
centripetal force
needed to hold a stone
in a sling, and the
relation between the
length of a pendulum
and the time of its
swing book: Principia
I (1687)
Newton’s First law of motion
 Also known as law of inertia,
 States,
– An object will remain at rest, or uniform
motion in a straight line, with the same speed
and in the same direction unless acted upon by
an unbalanced force.
Newton’s First law of motion
 Comments
– This means that if you
leave a book on a
bench over night, when
you return in the
morning, unless an
outside force moved it,
it will be in the same
place
No external forces applied-> the
book remains at rest
Newton’s First law of motion
e bench pushes upward
on the book
 Comments & Examples
– But what is an unbalanced
force? first consider a book
at rest on a bench. There are
two forces acting upon the
book. - the Earth's
gravitational force, and the
push of the bench on the
book (sometimes referred to
Gravity pulls downward on
as a Fn). Since these two
the book
forces are of equal
magnitude and in opposite
directions, they balance
each other. The book is said
to be at equilibrium.
Newton’s First law of motion
 Comments & Examples
– Consider another example
of a balanced force. There
are two forces acting upon
this person; The force of
gravity and the force of the
floor. these two forces are
equal magnitude and in
opposite directions, The
person is at equilibrium.
Gravity pulls downward on
the person
The floor pushes upw
on the person
Newton’s First law of motion
Involving Friction
Comments & Examples
The bench pushes upward
on the book
Gravity pulls downward on
the book
Force of friction
between the
bench/book
Now consider a book sliding
from right to left across a
bench. Sometime in the
prior history of the book, it
may have been given a
shove. The force of gravity
and the force of the bench
on the book balance each
other. Yet there is no force
present to balance the force
of friction. As the book
moves to the left, friction
acts to the right to slow the
book down. There is an
unbalanced force. The book
is not at equilibrium and
subsequently accelerates
Newton’s First law of motion
Involving Friction
 Let’s exercise
– Consider that the book weighs 0.2 kg. As it slides
across the bench with a constant velocity, its coefficient
of friction is 0.15. What force must be exerted on the
book, so that it maintains its constant velocity? (go to the
next slide for the answer)
Fn
Fob = ?
Ffr
Fg
Newton’s First law of motion
Involving Friction
 Answer & explanations
– We know that the magnitude of the force of gravity is mg. We
recognize that the two object in contact are in relative motion
(kinetic friction = Ffr = μkFn).
– Solving with the y-direction equation gives Fn = mg, and solving
for the x-direction, F
= μkmg)
– The force that must be used on the book is F
(0.2)(0.15)(9.80 m/s) = 0.294 N
= μkmg =
Newton’s First law of motion
 Comments &
Examples
– Considering a soccer
ball in the middle of a
field with no external
forces exerted (kicking,
moving, high
winds,…) on it.
Normal force of the
ground on the ball
No external
forces
Force of gravity
on the ball
Newton’s First Law of Motion
The floor pushes upward
on the person
 Comments &
Examples
Fn
Fg
– If you kick the soccer
ball, it will continue
moving until it hits
something.Newton’s
First Law of Motion
Gravity pulls downward on
the person
Newton’s First Law of Motion
The floor pushes upward
on the person
 Comments & Examples
Fn
Force of contact
between the
foot and the
ball
Fg
– Your foot can only interact
with the ball through forces
of contact (there is a
gravitational force between
your foot and the ball, but it
is so tiny that it is
completely negligible), so
once the ball is not in
contact with your foot, it no
longer exerts any force on
the ball.
Gravity pulls downward on
the person
Newton’s First law of motion
involving Friction
 Comments & Examples
Fn
Fn
Fg
Friction between
the ball and
the air
Fg
Ffr
– Once the ball is not in
contact with the foot, the
only object interacting with
the ball is the ground. The
ball will eventually stop
even if it does not hit a wall
(the friction between the
ball and the ground, and
between the ball and the
air)Newton’s First law of
motion
Newton’s First law of motion
 Comments &
Examples
– We feel the effects of
Newton's First Law
every day, but usually
don't notice them
because other forces
interfere. If it was not
for other forces we will
be in constant motion.
Newton’s First law of motion
 Comments &
Examples
Friction
between the
wind and the
plane
Direction of the
force due to
the reactors
Fg
Direction of the
force from
the reactors
– On earth, the
atmosphere will
eventually slow down
all moving objects, but
in a vacuum (basically
an empty space with no
air or atmosphere), like
space, it will be more
obvious that objects
obey Newton's Laws.
Newton’s First law of motion
 Comments &
Examples
– In space, the First Law
is much more obvious.
Objects will follow
their natural
trajectories until they
are stopped by an
outside force.
Newton’s First law of motion

 Comments & Examples
– One of the most common
places people feel the First
Law is in a fast moving
vehicle, such as a car or a
bus, that comes to a stop.
An outside force stops the
vehicle, but the passengers,
who have been moving at a
high speed, are not stopped
and continue to move at the
same speed
Newton’s First law of motion
 Comments & Examples
– If the car hits a cement road
divider it is stopped
(outside force). The crash
dummy, however is not so
lucky. Since he is not
wearing a seat belt, and is
not connected to the car, he
will continue to move at 60
mph, flying out through the
front windshield.
Newton’s First Law of Motion
 Comments & Examples
– The dummy will fly
through the air until he hits
the ground. This is because
the earth's gravity stopped
him from moving any
further. If this collision had
happened in zero-g, in a
vacuum, the dummy would
theoretically keep on
hurtling away from the car
at 60 mph.
Newton’s Second law of motion
 States,
– The acceleration of an object is directly
proportional to the net force acting on it and is
inversely proportional to its mass.
– The direction of the acceleration is in the
direction of the net force acting on the object
Newton’s Second law of motion
shortened => ΣF = ma
– where f is a push or pull that gives energy to an object
the motion of the object. a is the rate of change of
velocity.
Newton’s Second law of motion
Heavy mass, needs more force
 Comment & Example
– Newton's Second Law
is more abstract than
the first. The greater
the mass, the greater
the amount of force
needed to accelerate
the object.
Small mass, needs less force
Newton’s Second law of motion
 Example:
– Betty is developing her muscles by pushing this car that
weighs 1500 kg. She makes it go 0.02 m/s/s. Using
Newton's Second Law, can you compute how much
force I applied to the car? (the answer in the next slide)
Not really who you
expect to push the
car !!!
Force exerted by the
ground on the car
F = mass car x g
Newton’s Second law of motion
Fn
30 Newton applied
Comments & Examples
Fg
Betty has not really move that much consider she has only
exerted 30 Newton of force. (F=MA, so you plug in the data and get
F = 1500kg x .02 m/s/s. This comes out to 30 kg m/s/s, which is equal to 30
Newton.
Newton’s Second law of motion
 Example:
– Here Betty is trying to do the impossible. She wants to
push this 2500 kg van to a gas station. She computes
125 Newton on the car. How fast will she make it go?
She’s trying hard !!!
Force exerted by the
ground on the car
125 N
A=?
F = 2500 x g
Newton’s Second law of motion
Fn
125 N
0.05 m/s/s
Fg
Answer & Explanations
– It may seem impossible but Betty will make it go 0.5
m/s/s. Because using Newton's Second Law, we found
that… (F=MA, => A=F/M. So you plug in the data and get A = 125/2500kg.
This comes out to 0.05 m/s/s.
Newton’s Third law of motion
 Comments:
– Anytime an object exerts a force on another object, the
second object exerts an equal and opposite force on the
first.
Newton’s Third law of motion
 Comments
– Newton's Third Law is probably the most
famous of his laws.
– The Third Law at first seems simple, but is a
very important law.
– Every time we interact with our surroundings
we feel the Third Law.
Newton’s Third law of motion
 Comments &
Examples
Object A
Object B
– If use the convention
that F means the force
on object A from object
B, then Newton's third
law can be written
FAB = - FBA
Newton’s Third law of motion
 Comments &
Examples:
Force exerted on his face
by the punch
– When you punch
someone in his face
your hand not only
Force exerted on the hand applies a force to the
by his face
person's face, the
person's face applies a
force to your hand.
Newton’s Third law of motion
 Comments &
Examples
– The magnitude of the
force on each body is
identical and the forces
on the on the two
bodies are in the
opposite directions to
each other.
Ffp
-Fpf
Newton’s Third law of motion
 Comments &
Examples:
Force exerted on the rocket
by the engine
Force exerted on the engine
by the rocket
– The only reason why a
rocket is able to
launch, is that when its
engine pushes out the
gases, the gases exert
an equal and opposite
force back on the
rocket, which
accelerate.
Newton’s Third law of motion
 Comments &
Examples:
Force exerted on her foot by
the floor
Force exerted on the floor
by her foot
– One of the most
unnoticeable Newton’s
third law, is when we
walk.
– We can walk forward
because, when one foot
pushes backward
against the ground, the
ground pushes forward
on that foot.
Newton’s Third law of motion
The floor pushes
upward on the
person
 Comments &
Examples:
– Newton first law still
applying in this case.
– Her mass has also in
influence on her
walking.
Force exerted on her foot by
the floor
Gravity pulls
downward on
the person
Force exerted on the floor
by her foot
Newton’s Third law of motion
 Comments &
Examples:
– Even in the most
unthinkable moment,
we do exert Newton’s
third law.
– We cannot be touched
without being touched
The End
presented
by
BUENO OLIVIER